This invention relates to a composite feedback predictive code communication system for a color television signal including a carrier chrominance signal component. More particularly, this invention relates to a predictive encoder and a predictive decoder for use in a code communication system of the type described.
For code transmission of a television signal, an interframe predictive code communication system is already known wherein interframe or previous-frame feedback is resorted to on producing prediction error signals by using samples of a previous frame as predicted samples in view of the interframe correlation of the television signal. The interframe predictive code communication system is effective for a television signal for a slowly moving object. For raising the efficiency of the code transmission, a composite feedback predictive code communication system is also known wherein use is made of the interframe prediction and of intraframe prediction which may be either intersample or previous-sample feedback or interline or previous-line feedback or both. A conventional composite feedback predictive code communication system is effective for a monochrome television signal because encoding is carried out with the interframe redundancy removed by the interframe prediction and with the intraframe redundancy avoided by the intraframe prediction.
The present-day color television signals, such as NTSO, SECAM, and PAL signals, are in a form of a frequency-multiplexed color television signal comprising a luminance signal component and a carrier chrominance signal component. The latter component is produced by modulating a subcarrier by chrominance signals. For the subcarrier frequency, it is usual to select a frequency equal to an odd integral multiple of a half of the line frequency and consequently an odd integral multiple of a half of the frame frequency in order to reduce the undesirable degradation which would otherwise be introduced into the quality of the reproduced picture by interference between the luminance and the carrier chrominance signal components. The subcarrier therefore has a reversed phase during scan of the next succeeding line and also of the next succeeding frame. As will later be described more in detail, this inevitably amplifies the carrier chrominance signal components in the previous-line and previous-frame feedback prediction error signals by a factor of about two and makes it impossible to apply the interframe predictive and the composite feedback predictive code transmission directly to the frequency-multiplexed color television signals. The practice has therefore been to demodulate a frequency-multiplexed color television signal to baseband signals, such as the luminance signal Y and the chrominance signals I and Q in the case of an NTSC signal, to produce a plurality of predictive encoded signals by separately applying composite feedback predictive encoding to the respective baseband signals, and to multiplex the predictive encoded signals for transmission to a receiver or receivers. This renders it possible to make the best use of the salient features of composite feedback predictive encoding. It is, however, thereby unavoidable that the communication system is complicated, that the picture quality is degraded during demodulation and re-modulation, and that the communication system loses the compatibility to the monochrome television signals.